Cold-molding process for loading a stent onto a stent delivery system

ABSTRACT

A method of making a stent delivery system is provided in which a delivery catheter has a balloon that extends non-uniformly into interstices of a stent. In accordance with the method a balloon/stent/crimping tube assembly is placed in a crimping tool, the balloon is inflated, and the crimping tool is actuated to compress the stent on the outside of the balloon without application of heat or chemicals, thereby causing creases of the balloon to extend non-uniformly into the interstices of the stent. Optionally, pillows may be formed in the balloon to prevent longitudinal movement of the stent with respect to the balloon during intravascular delivery. One or more secondary crimpings also may be performed to achieve a smoother delivery profile.

FIELD OF THE INVENTION

The present invention relates to a cold-molding process for loading astent onto a stent delivery system. More specifically, the presentinvention relates to a method of loading a stent onto a balloon havingcreases that extend non-uniformly into the interstices of the stentwithout the use of a heating step.

BACKGROUND OF THE INVENTION

A stent is commonly used alone or in conjunction with angioplasty toensure patency through a patient's stenosed vessel. Stents overcome thenatural tendency of the vessel walls of some patients to restenose afterangioplasty. A stent is typically inserted into a vessel, positionedacross a lesion, and then expanded to create or maintain a passagewaythrough the vessel, thereby restoring near-normal blood flow through thevessel.

A variety of stents are known in the art, including self-expandable andexpandable stents, as well as wire braid stents. One such stent isdescribed, for example, in U.S. Pat. No. 4,733,665 to Palmaz. Expandablestents are typically delivered to treatment sites on delivery devices,such as balloon catheters or other expandable devices. Balloon cathetersmay comprise a balloon having a collapsed delivery configuration withwings that are wrapped and folded about the catheter. An expandablestent is then disposed in a collapsed delivery configuration about theballoon by compressing the stent onto the balloon. The stent and balloonassembly may then be delivered, using well-known percutaneoustechniques, to a treatment site within the patient's vasculature, forexample, within the patient's coronary arteries. Once the stent ispositioned across a lesion at the treatment site, it is expanded to adeployed configuration by inflating the balloon. The stent contacts thevessel wall and maintains a path for blood flow through the vessel.

Significant difficulties have been encountered during stent delivery anddeployment, including difficulty in maintaining the stent on the balloonand in achieving symmetrical expansion of the stent when deployed.Several techniques have been developed to more securely anchor the stentto the balloon and to ensure more symmetrical expansion. These includeplastically deforming the stent so that it is crimped onto the balloon,and sizing the stent such that its internal diameter provides aninterference fit with the outside diameter of the balloon catheter. Suchtechniques have several drawbacks, including less than optimalsecurement of the stent to the balloon. Consequently, the stent maybecome prematurely dislodged from the balloon during advancement of thestent delivery system to the treatment site.

Stent delivery systems utilizing a removable sheath disposed over theexterior surface of the stent, which is removed once the stent ispositioned at the treatment site, have also been proposed, for example,in U.S. Pat. No. 5,690,644 to Yurek et al. Such systems may be used withor without retainer rings and are intended to protect the stent duringdelivery and to provide a smooth surface for easier passage through thepatient's vasculature. However, the exterior sheath increases thecrossing profile of the delivery system while decreasing flexibility,thereby decreasing the ability of the device to track through narrowedand tortuous anatomy.

U.S. Pat. No. 6,106,530 to Harada describes a stent delivery devicecomprising a balloon catheter having stoppers disposed proximal anddistal of a balloon on to which a stent is affixed for delivery. Thestoppers are separate from the balloon and maintain the stent's positionin relation to the balloon during delivery. As with the removablesheaths discussed previously, the stoppers are expected to increasedelivery profile and decrease flexibility of the stent/balloon system.

U.S. Pat. No. 6,110,180 to Foreman et al. provides a catheter with aballoon having pre-formed, outwardly-extending protrusions on theexterior of the balloon. A stent may be crimped onto the balloon suchthat the protrusions extend into the gaps of the stent, thereby securingthe stent about the balloon for delivery. A drawback to this device isthe added complexity involved in manufacturing a balloon with pre-formedprotrusions. Additionally, if the protrusions are not formed integrallywith the balloon, there is a risk that one or more of the protrusionsmay detach during deployment of the stent. The protrusions may alsoreduce flexibility in the delivery configuration, thereby reducingability to track through tortuous anatomy.

U.S. Pat. No. 5,836,965 to Jendersee et al. describes a hot-moldingprocess for encapsulating a stent on a delivery system. Encapsulationentails placement of the stent over a balloon, placement of a sheathover the stent on the balloon, and heating the pressurized balloon tocause it to expand around the stent within the sheath. The assembly isthen cooled while under pressure to cause the balloon to adhere to thestent and to set the shape of the expanded balloon, thereby providingsubstantially uniform contact between the balloon and the stent. Thismethod also provides a substantially uniform delivery profile along thesurface of the encapsulated balloon/stent assembly.

A significant drawback of Jendersee's encapsulation method is the needto heat the balloon in order to achieve encapsulation. Such heatingwhile under pressure may lead to localized plastic flows resulting ininhomogeneities along the length of the balloon including, for example,varying wall thickness. Varying wall thickness may, in turn, yield areasof decreased strength that are susceptible to rupture upon inflation ofthe balloon during deployment of the stent. Additionally, heating andcooling increases the complexity, time, and cost associated withaffixing the stent to the balloon.

U.S. Pat. No. 5,976,181 to Whelan et al. provides an alternativetechnique for stent fixation involving the use of solvents to soften theballoon material. In this method, the stent is disposed over anevacuated and wrapped balloon while in its compact deliveryconfiguration. A rigid tube is then placed over the stent and balloonassembly, and the balloon is pressurized while the balloon is softenedby application of a solvent and/or heating. The rigid tube prevents thestent from expanding but allows the balloon to deform so that itssurface projects through either or both of the interstices and ends ofthe stent. Softening under pressure molds the balloon material such thatit takes a permanent set into the stent. Once pressure is removed, thestent is interlocked with the surface of the balloon, providingsubstantially uniform contact between the balloon and the stent and asubstantially uniform delivery profile.

As with the technique in the Jendersee patent, the technique in theWhelan patent has several drawbacks. Chemically softening the balloonmaterial under pressure is expected to introduce inhomogeneities alongthe length of the balloon, such as varying wall thickness, which againmay lead to failure of the balloon. Additionally, chemical alteration ofthe balloon, via application of a solvent to the surface of the balloon,may unpredictably degrade the mechanical characteristics of the balloon,thereby making accurate and controlled deployment of a stent difficult.Softening also adds cost, complexity, and time to the manufacturingprocess.

In view of the drawbacks associated with previously known methods andapparatus for loading a stent onto a stent delivery system, it would bedesirable to provide methods and apparatus that overcome thosedrawbacks.

It would be desirable to provide methods and apparatus for loading astent onto a stent delivery system that enhance positional stability ofthe stent during delivery.

It would further be desirable to provide methods and apparatus forloading a stent onto a stent delivery system wherein the delivery systemcomprises a crossing profile and flexibility suitable for use intortuous and narrowed anatomy.

It would still further be desirable to provide methods and apparatus forloading a stent onto a stent delivery system that provide asubstantially symmetrical expansion of the stent at deployment.

It would also be desirable to provide methods and apparatus for loadinga stent onto a stent delivery system that do not unpredictably modifythe mechanical characteristics of the balloon during fixation of thestent to the balloon.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide methods and apparatus for loading a stent onto a stent deliverysystem and deployment that overcome drawbacks associated with previouslyknown methods and apparatus.

It is an object to provide methods and apparatus for loading a stentonto a stent delivery system that enhance positional stability of thestent during delivery.

It is an object to provide methods and apparatus for loading a stentonto a stent delivery system wherein the delivery system comprises acrossing profile and flexibility suitable for use in tortuous andnarrowed anatomy.

It is also an object to provide methods and apparatus for loading astent onto a stent delivery system that provide a substantiallysymmetrical expansion of the stent at deployment.

It is an object to provide methods and apparatus for loading a stentonto a stent delivery system that do not unpredictably modify themechanical characteristics of the balloon during fixation of the stentto the balloon.

These and other objects of the present invention are achieved byproviding methods and apparatus for cold-molding a stent to the balloonof a stent delivery system so that the balloon extends non-uniformlyinto the interstices of the stent. In a preferred embodiment, the stentis a balloon expandable stent and is manufactured in a fully-expandedstate or in an intermediate-expanded state (i.e., having a diametersmaller than its fully-expanded, deployed diameter, but larger than itscompressed delivery diameter).

The stent is disposed on the balloon of a delivery catheter, and theballoon and stent are placed within an elastic crimping tube. Theballoon/stent/crimping tube assembly is then placed in a crimping tool,and the balloon is inflated, preferably only partially. The crimpingtool is actuated to compress the stent on the outside of the partiallyinflated balloon and to cause creases of the balloon to extendnon-uniformly into the interstices of the stent. Crimping occurs at asubstantially constant temperature, without the use of chemicals. Theballoon is then deflated, and the elastic crimping tube is removed.

Optionally, pillows or bumpers may be formed in the proximal and/ordistal regions of the balloon during crimping that, in conjunction withthe non-uniform creases of the balloon, prevent longitudinal movement ofthe stent with respect to the balloon during intravascular delivery.

Furthermore, one or more additional, secondary crimping steps may beperformed to achieve a smoother delivery profile, in which a semi-rigidcrimping tube is disposed over the stent delivery system, and theassembly is again disposed within the crimping tool. During secondarycrimping, the crimping tool is actuated to further compress the stentonto the unpressurized balloon. Secondary-crimping may alternatively beperformed with the balloon partially or completely pressurized/inflated.

Apparatus of the present invention may be used with a variety of priorart stents, such as balloon expandable stents, and may include tubularslotted stents, connected stents, articulated stents, multiple connectedor non-connected stents, and bi-stable stents. In addition to methods ofproduction, methods of using the apparatus of the present invention areprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention, its nature and various advantageswill be more apparent from the following detailed description of thepreferred embodiments, taken in conjunction with the accompanyingdrawings, in which like reference numerals refer to like partsthroughout, and in which:

FIGS. 1A-1C are, respectively, a side view of a stent delivery system inaccordance with the present invention, a cross-sectional view of thesystem along section line A-A in FIG. 1A, and a detail view of theballoon of the system non-uniformly extending within the interstices ofthe stent;

FIG. 2 is a flow chart showing the steps of the cold-molding process ofthe present invention;

FIGS. 3A-3C are, respectively, a side view of the distal end of thedelivery catheter of the system of FIGS. 1 in an expanded configuration,and cross-sectional views of the catheter along section line B-B in FIG.3A, showing the balloon evacuated to form radially extended wings and ina contracted configuration with the radially extended wings wrappedabout the catheter;

FIGS. 4A-4C are,respectively, a side view, partially in section, of thewrapped delivery catheter of FIG. 3C having the stent of FIGS. 1 and anelastic crimping tube disposed thereover, the entire assembly disposedwithin a crimping tool; a cross-sectional view of the same along sectionline C-C in FIG. 4A; and a detail view of the expandable structure ofthe stent;

FIGS. 5A and 5B are, respectively, a cross-sectional view along sectionline C-C in FIG. 4A of the apparatus upon pressurization of the balloon,and a detail view of the expandable structure of the stent;

FIG. 6 is a cross-sectional view along section line C-C in FIG. 4Aduring crimping after pressure has been removed;

FIG. 7 is a cross-sectional view along section line C-C in FIG. 4A of apossible configuration of the stent delivery system after crimping andremoval of the elastic crimping tube;

FIG. 8 is a side view, partially in section, of the stent deliverysystem disposed within a semi-rigid crimping tube and within thecrimping tool for optional secondary crimping; and

FIGS. 9A-9D are side views, partially in section, of the stent deliverysystem of FIGS. 1 disposed within a patient's vasculature, depicting amethod of using the apparatus in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises methods and apparatus for cold-molding astent onto a stent delivery system. More specifically, the presentinvention provides methods and apparatus for obtaining a balloon havingcreases that extend non-uniformly into the interstices of a stent loadedonto the exterior of the balloon, without the use of a heating orchemical process.

With reference to FIGS. 1, apparatus in accordance with the presentinvention is described. As seen in FIG. 1A, stent delivery system 10,illustratively shown in a collapsed delivery configuration, comprisesballoon expandable stent 20 loaded on balloon 14 of delivery catheter12. Stent 20 comprises an illustrative balloon expandable stent and maybe replaced with other stents known in the art. As seen in FIGS. 1B and1C, balloon 14 has creases 16 that extend non-uniformly into interstices22 of stent 20.

In FIG. 1B, creases 16 are shown with varying slope and height about thecircumference of stent delivery system 10. FIG. 1C depicts creases 16 asshaded areas and illustrates that creases 16 extend along the length ofstent 20 within interstices 22. Line L indicates the longitudinal axisof stent 20 in FIG. 1C. It should be understood that creases 16typically do not extend within every interstice 22 of stent 20.

Delivery catheter 12 preferably includes markers 17 disposed distal ofand proximal to stent 20 that facilitate placement of stent 20 onballoon 14, and that facilitate positioning of stent delivery system 10at a treatment site within a patient's vasculature. Markers 17 arepreferably radiopaque and fabricated from a radiopaque material, such asplatinum or gold. Catheter 12 preferably also comprises guide wire lumen13 and inflation lumen 15, which is coupled to balloon 14. As describedhereinbelow, during the cold-molding process of the present invention,proximal and/or distal pillows 19 optionally may be formed in balloon 14during pressurized crimping. As with creases 16, pillows 19 act toreduce or prevent longitudinal movement of the stent on the balloonduring intravascular delivery.

Balloon 14 is expandable by injection of a suitable medium, such as airor saline, via inflation lumen 15. Balloon 14 preferably expands stent20 to a deployed configuration under application of pressure in therange of about 6-9 atm. Additionally, balloon 14 preferably has a ratedburst pressure above 10 atm, and even more preferably between about12-14 atm. Balloon 14 may be fabricated from a variety of materials,including Nylon, polyethylene terephalate, polyethylene, andpolyether/polyamide block copolymers, such as PEBAX.

Additionally, balloon 14 may be fabricated from an elastomeric polyesterblock copolymer having an aromatic polyester hard segment and analiphatic polyester soft segment, such as “Pelprene,” which is marketedby the Toyobo Corporation of Osaka, Japan. Balloon 14 also may befabricated from a copolymer having a polybutylene terephalate hardsegment and a long chain of polyether glycol soft segment, such as“Hytrel” from the DuPont Corporation of Wilmington, Del.

Illustrative stent 20 may be fabricated from a variety of materials,including polymers and metals, and may comprise any of a variety ofprior art stents, such as balloon expandable stents, including tubularslotted stents, connected stents, articulated stents, multiple connectedor non-connected stents, and bi-stable stents. Stent 20 also may includeexternal coating C configured to retard restenosis or thrombus formationin the vessel region surrounding the stent. Alternatively, coating C maydeliver therapeutic agents into-the-patient's blood stream or vesselwall.

Referring now to FIGS. 2-8, a method of producing stent delivery system10 is described. FIG. 2 provides an overview of the cold-molding processof the present invention, while FIGS. 3-8 provide detailed views ofthese process steps.

As depicted in FIG. 2, the cold-molding process of the present inventioninvolves steps of: obtaining a stent, step 102; obtaining a ballooncatheter, step 103; disposing the stent on the balloon of the ballooncatheter, step 104; and disposing an elastic crimping sleeve over thestent and balloon, step 105. In accordance with the method of thepresent invention, the balloon is then inflated—preferably onlypartially—with an inflatable medium, such as air, at step 106. Thesleeve/stent/balloon assembly is then crimped within a crimping toolthat compresses the stent onto the balloon, step 107, while the balloonis pressurized.

As described hereinbelow, this step causes the balloon to bulge into theinterstices of the stent, and in addition, to form pillows 19, proximalof, and distal to, the ends of the stent to retain the stent in placeduring transluminal delivery. At step 108, the balloon is depressurized,and the elastic sleeve is removed to complete the stent loading process.

If desired, a semi-rigid sleeve optionally may be disposed over thestent/balloon assembly, and one or more additional crimping steps may beperformed, steps 109 and 110 of FIG. 2.

Referring now to FIGS. 3-8, additional details of a preferred embodimentof the process of the present invention are illustrated and described.In FIGS. 3, balloon 14 of delivery catheter 12 preferably is foldedprior to placement of stent 20 about balloon 14. Balloon 14 is firstexpanded, as in FIG. 3A, and then evacuated to form radially extendedwings 18, as seen in FIG. 3B. Balloon 14 is illustratively depicted withfour wings 18, but it should be understood that any number of wings maybe provided, for example, two, three or five wings. In FIG. 3C, wings 18are wrapped about the shaft of delivery catheter 12 to dispose catheter12 in a contracted configuration. It should be understood that balloon14 may alternatively be folded and/or disposed in a collapsed deliveryconfiguration by other techniques, for example, with techniques that donot utilize wings.

With reference to FIGS. 4, stent 20 and elastic crimping tube 30 aredisposed about balloon 14, preferably with stent 20 positioned betweenmarkers 17 of delivery catheter 12 (steps 102-105, FIG. 2). Theballoon/stent/crimping tube assembly is inserted within crimping tool40, as seen in FIG. 4A. Crimping tool 40 is preferably positionedbetween markers 17 to facilitate formation of optional pillows 19 duringpressurization of balloon 14. Crimping tool 40 may be any of a varietyof crimping tools known in the art. An illustrative crimping tool isdescribed, for example, in U.S. Pat. No. 6,082,990 to. Jackson et al.,which is incorporated herein by reference.

Referring to FIG. 4B, stent 20 may be directly placed about balloon 14,and elastic crimping tube 30 then may be loaded over the stent/balloonassembly. Alternatively, stent 20 may be placed within elastic crimpingtube 30, and then the stent/tube assembly disposed surrounding balloon14. As yet another alternative, crimping tube 30, or crimping tube 30and stent 20, may be positioned within crimping tool 40; then, balloon14, with or without stent 20 loaded thereon, may be positioned withincrimping tool 40.

As depicted in FIG. 4C, stent 20 preferably is manufactured in anintermediate-expanded state having a diameter smaller than its expandeddeployed diameter, but larger than its compressed delivery diameter,thereby facilitating positioning of stent 20 about balloon 14. Whenstent 20 is initially disposed surrounding balloon 14, the balloon doesnot substantially extend into interstices 22 of stent 20. It should beunderstood that stent 20 alternatively may be manufactured in afully-expanded state.

In FIGS. 5, once stent 20 and crimping tube 30 are disposed aboutballoon 14 of delivery catheter 12, and once the entire assembly isdisposed within crimping tool 40, balloon 14 is pressurized, forexample, via an inflation medium delivered through inflation lumen 15 ofcatheter 12 (step 106, FIG. 2). Pressure application causes balloon 14to enter a portion of interstices 22 of stent 20 in a non-uniformmanner, as seen in the cross section of FIG. 5A and in the detail viewof FIG. 5B. Crimping tube 30 and crimping tool 40 prevent expansion ofstent 20 during partial or complete pressurization of balloon 14, asdepicted in FIG. 5A.

The inflation medium is preferably delivered at a pressure in the rangeof about 6-8 atm. This pressure range is below the preferred rated burstpressure of balloon 14, which is above 10 atm, and even more preferablybetween about 12-14 atm, and thus ensures that the balloon does notpuncture. The elasticity of crimping tube 30 allows the tube to expandslightly upon application of pressure, and to contract slightly duringcrimping. Tube 30 may be fabricated from any suitable elastic material,for example, a polymer, such as PEBAX. Elastic crimping tube 30preferably has a hardness of between about 30 and 40 Shore Hardness, andmore preferably a hardness of about 35 Shore Hardness.

With reference to FIG. 6, in conjunction with FIG. 4A, crimping tool 40is actuated to crimp stent 20 onto balloon 14 (step 107, FIG. 2).Crimping tool 40 applies an inwardly-directed stress, σ_(crimp), to theassembly. Initially, balloon 14 is still pressurized. Stent 20 iscompressed onto the outside of balloon 14, causing the balloon tofurther bulge non-uniformly into interstices 22 of the stent. Crimpingpreferably proceeds along the length of the balloon/stent/tube assemblyall at once but may alternatively proceed in sections, so that theassembly is gradually crimped along its length.

Balloon 14 is then depressurized, allowing crimping tool 40 to furthercompress stent 20 onto balloon 14, as seen in FIG. 6 (step 108, FIG. 2),which forms creases 16 of balloon 14 that extend non-uniformly withininterstices 22 of the stent. Creases 16 are most clearly seen in FIGS.1B and 1C. Optional pillows 19 of stent delivery system 10 are alsoformed. Since many prior art crimping tools 40 apply aninwardly-directed stress, σcrimp, that is not uniform about the radiusof balloon 14, elastic crimping tube 30 acts to more uniformlydistribute the stress about the circumference of the balloon/stentassembly.

Stent delivery system 10 is removed from elastic crimping tube 30 andcrimping tool 40 (step 108, FIG. 2). Stent delivery system 10 has alow-profile delivery configuration adapted for percutaneous deliverywithin a patient's vasculature, as described hereinbelow with respect toFIGS. 9. Creases 16, as well as pillows 19, secure stent 20 to balloon14 between markers 17 of delivery catheter 12.

In contrast to prior art techniques described hereinabove, crimping inaccordance with the present invention occurs at a substantially constanttemperature, without the use of chemicals. In the context of the presentinvention, substantially constant temperature during crimping should beunderstood to include minor fluctuations in the actual temperature dueto frictional losses, etc.

Importantly, the system of the present invention is not actively heatedto thermally remodel the balloon, as described in U.S. Pat. No.5,836,965 to Jendersee et al. Likewise, no solvents are added to softenand mold the balloon, as described in U.S. Pat. No. 5,976,181 to Whelanet al. As described previously, both heating and solvents havesignificant potential drawbacks, including inhomogeneities along thelength of the balloon, such as varying wall thickness. Varying wallthickness may yield areas of decreased strength that are susceptible torupture upon inflation of the balloon during deployment of the stent.Additionally, heating and cooling, as well as addition of solvents,increases the complexity, time, and cost associated with affixing thestent to the balloon.

Theoretical bounds for the radial stress that may be applied to balloon14 during crimping, while the balloon is pressurized, may be estimatedby modeling balloon 14 as an idealized tube and assuming crimping tool40 applies an evenly distributed, inwardly-directed radial stress,scrimp. Stent 20 and elastic crimping tool 30, meanwhile, theoreticallyresist the crimping stress with an outwardly-directed radial stress,σ_(resistance). Thus, the composite inwardly-directed radial stress,σ_(in), applied to balloon 14 may be idealized as:σ_(in)=σ_(crimp)−σ_(resistance)   (1)Pressurization/inflation of balloon 14 similarly may be modeled as anevenly distributed, outwardly-directed radial stress, σ_(o) and it maybe assumed that the rated burst pressure of balloon 14 is the yieldstress of the balloon, σ_(y). A stress balance provides:σ_(in)−σ_(out)<σ_(y)   (2)Thus, a theoretical upper bound for the radial stress, σ_(in), that maybe applied to balloon 14 is:σ_(in)<σ_(y)+σ_(out)   (3)A theoretical lower bound for σ_(in) also may be found by observingthat, in order to compress stent 20 onto the exterior of balloon 14,crimping tool 40 must apply a radial stress, σ_(crimp), that is greaterthan the net stress provided by resistance of stent 20 and crimping tube30, σ_(resistance), and by the inflation of balloon 14, σ_(out):σ_(crimp)>σ_(out)+σ_(resistance)   (4)Combining Equation (1) and (4) provides a lower bound for σ_(in):σ_(in)>σ_(out)   (5)Finally, combining Equations (3) and (5) provides a range for σ_(in):σ_(out)<σ_(in)<σ_(y)+σ_(out)   (6)

As an example, assuming a burst pressure, σ_(y), of 12 atm and a balloonpressurization, σ_(out), of 8 atm, the balloon will theoreticallywithstand an inwardly-directed stress, σ_(in), of up to 20 atm.Furthermore, in order to ensure that stent 20 is crimped onto balloon14, σ_(in) must be greater than 8 atm. Thus, the inwardly-directedradial stress must be between 8 and 20 atm. Assuming, for example, aresistance stress, σ_(resistance), of 2 atm, crimping tool 40 must applya crimping stress, σ_(crimp), between 10 and 22 atm. As one of ordinaryskill will readily understand, the actual radial stress applied shouldbe further optimized within this range to provide a safety factor,optimal crimping, etc. Since balloon 14 is not in reality an idealizedtube, stresses applied to the balloon will have a longitudinal componentin addition to the radial component, which may be, for example,accounted for in the safety factor.

With reference now to FIG. 7, a possible configuration of the stentdelivery system after crimping and removal of elastic crimping tube 30is described. One or more struts 21 of stent 20 may be incompletelycompressed against balloon 14. Such a strut may potentially snag againstthe patient's vasculature during delivery, and thereby preventpositioning of stent delivery system 10 at a treatment site.Additionally, pressurized crimping may result in a delivery profile fordelivery system 10 that is more polygonal than cylindrical, therebyapplying undesirable stresses on the vessel wall during transluminalinsertion. Accordingly, it may be desirable to perform an optionalsecondary crimping step after balloon 14 has been depressurized.

Referring to FIG. 8, in order to reduce the potential for incompletelycompressed individual struts 21 of stent 20, and to provide a moreuniform cylindrical delivery profile, one or more additional, secondarycrimping steps may be performed on stent delivery system 10. In FIG. 8,stent delivery system 10 is disposed within semi-rigid crimping tube 50,which is disposed within crimping tool 40 (step 109, FIG. 2). Tube 50may be fabricated from any suitable semi-rigid material. As with elasticcrimping tube 30, semi-rigid crimping tube 50 preferably comprises apolymer, such as PEBAX. Semi-rigid crimping tube 50 preferably has ahardness of between about 50 and 60 Shore Hardness, and more preferablya hardness of about 55 Shore Hardness.

With stent delivery system 10 disposed within semi-rigid tube 50 andcrimping tool 40, tool 40 is actuated to compress individual struts 21against balloon 14 and to give delivery system 10 the substantiallycylindrical delivery profile of FIG. 1B (step 110, FIG. 2). As withelastic crimping tube 30, semi-rigid tube 50 acts to evenly distributecrimping stresses applied by crimping tool 40 around the circumferenceof the stent/balloon assembly. Since balloon 14 is not pressurized,secondary crimping preferably proceeds in sections along the length ofstent delivery system 10. However, as will be apparent to those of skillin the art, secondary crimping may proceed in one step. Optionally,balloon 14 may be pressurized during secondary crimping.

Referring now to FIGS. 9, a method of using stent delivery system 10 ofthe present invention is described. Stent delivery system 10 is disposedin a contracted delivery configuration with stent 20 disposed overballoon 14 of delivery catheter 12. Creases 16 of balloon 14non-uniformly extend within interstices 22 of stent 20. Creases 16, inconjunction with optional pillows 19, act to secure stent 20 to balloon14. As seen in FIG. 9A, the distal end of catheter 12 is delivered to atarget site T within a patient's vessel V using, for example, well-knownpercutaneous techniques. Target site T may, for example, comprise astenosed region of vessel V. The radiopacity of markers 17 mayfacilitate positioning of system 10 at the target site. Alternatively,stent 20 or other portions of catheter 12 may be radiopaque tofacilitate positioning.

In FIG. 9B, balloon 14 is inflated, for example, via an inflation mediumdelivered through inflation lumen 15 of catheter 12. Stent 20 expands tothe deployed configuration in which it contacts the wall of vessel V attarget site T. Expansion of stent 20 opens interstices 22 of the stentand removes the non-uniform creases of balloon 14 from within theinterstices. Additionally, stent 20 has a diameter in the deployedconfiguration that is larger than the diameter of optional pillows 19,thereby facilitating removal of stent 20 from delivery catheter 12.Balloon 14 is then deflated, as seen in FIG. 9C, and delivery catheter12 is removed from vessel V, as seen in FIG. 9D.

Stent 20 remains in place within vessel V in the deployed configurationin order to reduce restenosis and recoil of the vessel. Stent 20 alsomay comprise external coating C configured to retard restenosis orthrombus formation around the stent. Alternatively, coating C maydeliver therapeutic agents into the patient's blood stream or a portionof the vessel wall adjacent to the stent.

Although preferred illustrative embodiments of the present invention aredescribed hereinabove, it will be evident to those skilled in the artthat various changes and modifications may be made therein withoutdeparting from the invention.

For example, stent delivery system 10 may be produced without usingelastic crimping tube 30. In this case, the stent/balloon assembly wouldbe loaded directly into crimping tool 40, which would limit expansion ofballoon 14 during pressurization. Likewise, semi-rigid crimping tube 50may be eliminated from the secondary crimping procedure. If crimpingtubes are not used, crimping tool 40 preferably applies aninwardly-directed stress that is substantially evenly distributed aboutthe circumference of the stent/balloon assembly.

Additionally, balloon 14 may be depressurized prior to crimping stent 20onto the balloon. This may be particularly beneficial when crimping longstents, for example, stents longer than about 50 mm. Pressurization ofballoon 14 may cause the balloon to increase in longitudinal length.When crimping a long stent 20 onto a correspondingly long balloon 14,this increase in balloon length is expected to be more significant, forexample, greater than about 1 mm.

If stent 20 is crimped onto balloon 14 while the balloon is pressured,significant stresses may be encountered along creases 16 after balloon14 is depressurized, due to contraction of the balloon back to itsshorter, un-inflated longitudinal length. These stresses may, in turn,lead to pinhole perforations of balloon 14. Thus, since pressurizationof balloon 14 causes the balloon to extend at least partially withininterstices 22 of stent 20 in a non-uniform manner, as seen in FIG. 5A,it is expected that crimping after depressurization will still establishcreases 16 of stent delivery system 10, in accordance with the presentinvention. Obviously, crimping after depressurization may be done withstents 20 of any length, not just long stents.

It is intended in the appended claims to cover all such changes andmodifications that fall within the true spirit and scope of theinvention.

1. A stent delivery system having a contracted delivery configurationand an expanded deployed configuration, the stent delivery systemcomprising: a stent; and a delivery catheter having an inflatableballoon, the stent disposed about the balloon, wherein the ballooncomprises creases that extend non-uniformly within interstices of thestent in the contracted delivery configuration.
 2. The stent deliverysystem of claim 1, wherein the inflatable balloon further comprisespillows disposed proximally and distally of the stent in the contracteddelivery configuration.
 3. The stent delivery system of claim 1, whereinthe delivery catheter further comprises markers.
 4. The stent deliverysystem of claim 3, wherein the markers are radiopaque.
 5. The stentdelivery system of claim 1, wherein the stent comprises an externalcoating.
 6. The stent delivery system of claim 5, wherein the coating isconfigured to retard restenosis or thrombus formation around the stent.7. The stent delivery system of claim 5, wherein the coating isconfigured to deliver therapeutic agents into a patient's blood streamor vessel wall when implanted within the patient's vasculature.
 8. Amethod of fabricating a stent delivery system comprising: providingapparatus comprising a stent and a delivery catheter having aninflatable balloon; disposing the stent about the inflatable balloon;placing the balloon and stent within a crimping tool; inflating theballoon; deflating the balloon; and actuating the crimping tool tocompress the stent on the outside of the balloon at a substantiallyconstant temperature, thereby securing non-uniform creases of theballoon within the interstices of the stent and forming the stentdelivery system.
 9. A method for stenting at a target site within apatient's vessel, the method comprising: providing a stent deliverysystem comprising a stent and a delivery catheter with an inflatableballoon, the stent disposed about the inflatable balloon, wherein theballoon has creases that extend non-uniformly within interstices of thestent; percutaneously delivering the stent delivery system to the targetsite within the patient's vessel in a contracted delivery configuration;and expanding the stent delivery system to an expanded deployedconfiguration, wherein the balloon is inflated, the interstices of thestent open, removing the creases of the balloon from the interstices,and the stent engages the target site.
 10. The method of claim 9 furthercomprising: deflating the balloon; and removing the delivery catheterfrom the patient's vessel.